This invention relates to materials which melt only at very high temperatures and, more specifically, to alloys which melt only at high temperatures and exhibit improved resistance to oxidation at such temperatures.
There is presently great need for materials capable of sustained mechanical use at temperatures greater than about 1500.degree. C. Such materials find use, for example, in the manufacture of turbine blades and other components of jet engines. Materials which can be employed in such uses must have very high melting points. Unfortunately, most high-melting materials rapidly oxidize in the environments to which they are often exposed. Carbon-carbon composite materials provide a good example of high melting materials which are rapidly oxidized at elevated temperatures. A major barrier to the utilization of carbon-carbon composites and similar materials in commercial high temperature applications has been the development of coatings or other treatments which can provide adequate protection from oxidation.
The tendency of these materials to oxidize at high temperatures has thus created great interest in protective coatings comprising a variety of metals, metalloids, and alloys, one such protective coating is silicon carbide, which is often used on structural elements composed of carbon-carbon composites. Silicon carbide is believed to protect such materials by forming a surface layer of protective silicon oxide scale. However, silicon carbide coatings fail to provide adequate oxidation protection at temperatures above about 1500.degree. C.
An other class of coatings for carbon-carbon composites and other high-temperature materials comprises iridium and iridium-containing alloys. Alloys comprising iridium are among the most promising materials for applications in high temperature environments, due in considerable part to iridium's relatively high (2454.degree. C.) melting point. However, elemental iridium is quite expensive compared with other materials employed in high temperature applications. In addition, iridium and many iridium-containing alloys can have associated with them a number of serious performance-related difficulties. For example, coatings comprising iridium may exhibit adhesion problems in high temperature environments with materials such as carbon-carbon composites. A more serious difficulty in using iridium-containing alloys is their degradative tendency to rapidly form gaseous iridium oxides, such as IrO.sub.2 and IrO.sub.3, at high temperatures.
It is known that the generation of gaseous iridium oxides can be minimized or eliminated by the formation of a protective metal oxide barrier on the surface of an iridium-containing galloy. For example, it is known that when aluminum is incorporated into such alloys, an Al.sub.2 O.sub.3 barrier layer can be generated on the alloy's surface at high temperatures. This alumina scale inhibits the formation of iridium oxides. However, prior alloys consisting of iridium and aluminum are known to form truly protective external Al.sub.2 O.sub.3 layers only when the concentration of aluminum in the alloy is greater than about 55 atomic percent (at %). The minimum concentration of aluminum which needs be present in a given alloy to produce an effectively protective oxide layer is known as the alloy's critical aluminum concentration. At aluminum concentrations lower than the critical aluminum concentration, iridium/aluminum alloys form cracked or porous Al.sub.2 O.sub.3 layers which fail to inhibit both the transport of oxygen and the degradative generation of gaseous iridium oxides resulting therefrom.
Because aluminum has a relatively low melting point (660.degree. C.), its incorporation into an alloy generally has a deleterious effect upon the alloy's melting point. For example, the critical aluminum concentration in an iridium-containing alloy significantly lowers the melting point of the alloy as compared with its non aluminum-containing counterpart. It is therefore greatly desired that the incorporation of aluminum into alloys intended for high temperature applications be reduced without reducing the resistance to degradation of these alloys.
It is therefore an object of this invention to provide alloys capable of advantageous, sustained use at high temperatures.
It is a further object of this invention to provide such high temperature alloys as inexpensively as practicable.
It is another object of this invention to provide high temperature alloy coatings with good adhesion to a wide variety of substrates.
It is a further object of this invention to provide such alloys with improved resistance to even harsh oxidizing environments. Further objects are to provide shaped bodies comprising such alloys for structural, mechanical and chemical use and to secure methods for their fabrication.